Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Canon 400D and Pentax I-10 side-by-side from the front, back and top in their relative dimensions. Canon 400D has external dimensions of 127 x 94 x 65 mm (5 x 3.7 x 2.56″) and weighs 556 g (1.23 lb / 19.61 oz) (including batteries). Pentax I-10 has external dimensions of 101 x 65 x 28 mm (3.98 x 2.56 x 1.1″) and weighs 153 g (0.34 lb / 5.40 oz) (including batteries).
Below you can see the front view size comparison of Canon 400D and Pentax I-10. Pentax I-10 is clearly the smaller of the two cameras. Its body is 26mm narrower, 29mm shorter and 37mm thinner than Canon 400D. As you can tell, actually this is not a fair size comparison since Pentax I-10 has a built-in lens but Canon 400D doesn't. Once you put a lens on 400D, its thickness will increase significantly.
Comparison image of Canon 400D and Pentax I-10 Size, Weight and External Dimensions - Front View
Now lets look at the top view comparison of Canon 400D and Pentax I-10.
Top View Comparison image of Canon 400D and Pentax I-10 Size
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Pentax I-10 is significantly lighter (403g ) than the Canon 400D which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on 400D, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither Canon EOS 400D (EOS Digital Rebel XTi / EOS Kiss Digital X) nor Pentax Optio I-10 provides any type of weather sealing in their body, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Pentax I-10's 2.70" LCD screen is slightly larger than Canon 400D's 2.5" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.